Richard, that's not quite right. Formatting or reformatting a drive destroys all of the data that it contains. After the HAP-Z1ES reformats the drive, probably to Ext3 or Ext4 (presumably because its internal operating system is Linux-based), music files would then be obtained by the HAP-Z1ES from your computer via the wifi or wired ethernet network connection that would exist between them (via your router). I presume the HAP-Z1ES would then offer a choice as to whether the files it obtains are to be stored on the external drive or on its internal drive.
Also, I looked further at the descriptions of the Paragon ExtFS program I cited earlier. Assuming it works as advertised, it looks like it would be an excellent solution, allowing the drive to be removed from the HAP-Z1ES and connected to your computer when and if desired, the data it contains remaining intact, and then used with the computer just like any other drive.
Alternatively, there are various free software tools available that would allow you to reformat the drive as Kal did, making it usable with your computer. But of course doing that would destroy any data that had been stored on it by the HAP-Z1ES.
Regards,
-- Al